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CS 102

CS 102. Control Structures. Overview. Need to have program statements that control execution flow Simple statements to branch execution based on conditions (If/Then/Else) Nested If statements And/Or/XOR Not Select Case statements Class variables. Conditional Statements.

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CS 102

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  1. CS 102 Control Structures

  2. Overview • Need to have program statements that control execution flow • Simple statements to branch execution based on conditions (If/Then/Else) • Nested If statements • And/Or/XOR • Not • Select Case statements • Class variables

  3. Conditional Statements • All of the previous VB.NET that you have been shown are imperative commands: • Do this, then that • Linear • Only branching is some of them are event handlers • It is essential to have conditional branching in your programs: • User input • Based on current machine condition • Other factors

  4. Conditional Example Select random number (1-100) Ask for their guess Tell them if it was low/high Is it right? No Yes Yay!!

  5. Conditional Structure • Boolean logic (True/False) • Can be complex statement • Often uses variables • Structure is: If <condition> Then <statement(s)> End If • Condition can use “And” and “Or” • Statement block can be as long and complex as needed • Must ALWAYS have an “End If” to match the If

  6. Example • As long as the left side resolves to True/False: If 3 * 2 = 6 Then MsgBox("It is = 6!!") End If • More complex: If ChkTest.Checked Then MsgBox("Text box is checked") End If • Other logic: If ChkTest.Checked And ChkTest2.Checked Then MsgBox("Text boxes are both checked") Else MsgBox("They are not both checked") End If

  7. Else And ElseIf • Else clause executes if the “If” clause fails • Else can have any statement(s) included in the block If Not ChkTest.Checked Then MsgBox("Text box 1 not checked") Else If ChkTest2.Checked Then MsgBox("Both are checked!") End If End If • Results can be understood in a chart

  8. Nested If • You can nest If statements as deeply as you want • Each If must have an “End If” to match it • Can nest as deeply as you want If ChkTest.Checked Then MsgBox("Text box 1 is checked") Else If ChkTest2.Checked Then MsgBox("Checkbox 2 is checked") Else If chktest3.checked Then MsgBox("Checkbox 3 is checked") End If End If End If

  9. Other Boolean Operators • ElseIf – Can combine both into one keyword • And – Both conditions must be true • Or – Only one condition may be true • Not – True if the condition is False, otherwise true • Xor – Exclusive Or. Only true if 1 (and only 1) of the conditions is true. Otherwise false

  10. Chart of Boolean Logic

  11. Chart Of Boolean Logic

  12. Some Other Useful Functions • String.Empty – Is true if a string has no data in it If txtMyText = String.Empty Then… • ToUpper and ToLower change the case of a string strTest = “Hello” strTest = strTest.ToUpper() • IsNumeric - Is a string expresion numeric strTest = "123abc“ MsgBox(IsNumeric(strTest)) • Length – Returns the length of a string MsgBox(“Length is: “ & strTest.Length)

  13. Some Other Useful Functions • TrimStart, TrimEnd, Trim – Trim whitespace from a string • Substring – Returns a string from the middle of another string • Multiple versions of this function. Check documentation • IndexOf – Returns an integer with the starting location of a substring within a string strTest = “123abc” MsgBox("Start location is:" & strTest.IndexOf("ab"))

  14. Select Case • Complex logic with If/Else can get long and complex • Sometimes you just want to find the one case (of many) that is true • You can use the “Select Case” statement for this • It evaluates each of the cases in turn • The first one that is true is executed • It then continues on to the next case and evaluates it • Can exit the Select Case statement with

  15. Example Select Case CInt(TxtCheck.Text) Case 0 MsgBox("A zero?? Hard to believe") Case 1 To 59 MsgBox("That's a fail :(") Case 60 To 69 MsgBox("D") Case 70 To 79 MsgBox("C") Case 80 To 89 MsgBox("B") Case 90 To 99 MsgBox("A") Case 100 MsgBox("Wow! Perfect!!") Case Else MsgBox("Not a valid score. Try again") End Select

  16. Select Case • Can have individual items • Case 0 • Can have a range of items • Case 3, 4 • Can have a catchall (to get everything else) • Case Else • Select Case can have a variety of types • Integers • Strings • Virtually any other type

  17. Class Variables • All variables have a scope. That is, the part of the program for which the variable is recognized • Variables declared in a Subroutine are only defined for that Sub/Function • Class variables are declared at the top of the class, and can be accessed anywhere in that class • If you need access to the variable, you can pass the information in one of two ways: • Input/output argument • Class variables

  18. Class Variables • Declare a class variable Public Class Form1 Dim aVar as Integer ‘ This variable can be used anywhere in the class • Pass an input variable mySub(44) Private Sub mySub(ByVal intMyInt As Integer) MsgBox(intMyInt) End Sub

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